Understanding
SNMP

SNMP is an
application–layer communication protocol that allows network devices to
exchange management information among these systems and with other devices
outside the network. Through SNMP, network administrators can manage network
performance, find and solve network problems, and plan network growth. SNMP
makes network monitoring more cost effective and allows your network to be more
reliable.

CPT supports SNMP
Version 1 (SNMPv1), SNMP Version 2c (SNMPv2c), and SNMP Version 3 (SNMPv3). As
compared to SNMPv1, SNMPv2c includes additional protocol operations and 64–bit
performance monitoring support. SNMPv3 provides authentication, encryption, and
message integrity and is more secure.

Note

SNMP Version 3 (SNMPv3) is not supported for PTF and line cards.

The following figure
illustrates the basic layout idea of an SNMP-managed network.

Figure 1. Basic Network
Managed by SNMP

The advantages of SNMP
are as follows:

SNMP is LAN based.

SNMP is an open
standard.

SNMP can be easily
extended.

SNMP provides a
common management platform for many different devices.

Understanding SNMP Components

An SNMP–managed network consists of a manager, agents, and managed devices.

The manager provides the interface between the human network manager and the management system. The agent provides the interface between the manager and the physical device being managed

Management systems execute most of the management processes and provide the bulk of memory resources used for network management. A network might be managed by one or several management systems.

The following figure illustrates the relationship between the network manager, the SNMP agent, and the managed devices.

Figure 2. Example of the Primary SNMP Components

An agent residing on each managed device translates local management information data—such as performance information or event and error information—caught in software traps, into a readable form for the management system.

The following figure illustrates SNMP agent get–requests that transport data to the network management software.

Figure 3. Agent Gathering Data from a MIB and Sending Traps to the Manager

The SNMP agent captures data from MIBs, which are device parameter and network data repositories, or from error or change traps.

A managed element—such as a router, access server, switch, bridge, hub, computer host, or network element—is accessed through the SNMP agent. Managed devices collect and store management information, making it available through SNMP to other management systems having the same protocol compatibility.

Note

It is recommended that the SNMP Manager timeout value be set to 60 seconds. Under certain conditions, if this value is lower than the recommended time, the TNC/TSC card can be reset. However, the response time depends on various parameters such as object being queried, complexity, number of hops in the node and so on.

Understanding MIB

The Management Information Base (MIB) is a data structure that describes SNMP network elements as a list of data objects. The SNMP manager must compile the MIB file for each equipment type in the network to monitor SNMP devices.

The manager and agent use a MIB and a relatively small set of commands to exchange information. The MIB is organized in a tree structure with individual variables being represented as leaves on the branches. A long numeric tag or object identifier (OID) is used to distinguish each variable uniquely in the MIB and in SNMP messages. The MIB associates each OID with a readable label and various other parameters related to the object. The MIB then serves as a data dictionary or codebook that is used to assemble and interpret SNMP messages.

When the SNMP manager wants to know the value of an object, such as the state of an alarm point, the system name, or the element uptime, it will assemble a GET packet that includes the OID for each object of interest. The element receives the request and looks up each OID in its code book (MIB). If the OID is found (the object is managed by the element), a response packet is assembled and sent with the current value of the object included. If the OID is not found, a special error response is sent that identifies the unmanaged object.

Understanding SNMP Community Names

Community names are used to group SNMP trap destinations. All the trap destinations can be provisioned as part of SNMP communities in CTC. When community names are assigned to traps, the request is treated as valid if the community name matches one that is provisioned in CTC. In this case, all agent-managed MIB variables are accessible to that request. If the community name does not match the provisioned list, SNMP drops the request.

Accessing Fabric Card Through SNMP

Each fabric card runs a separate instance of SNMP. SNMP requests are relayed to the individual fabric card based on the community string. The community string uses the following format:

com_str_configured_from_CTC@fabric_card_slot_number

Understanding SNMP Messages

SNMP uses the following messages to communicate between the manager and the agent.

Get

GetNext

GetResponse

Set

Trap

The Get and GetNext messages allow the manager to request information for a specific variable. The agent, upon receiving a Get or GetNext message, will issue a GetResponse message to the manager with either the information requested or an error indication as to why the request cannot be processed.

A Set message allows the manager to request a change be made to the value of a specific variable in the case of an alarm remote that will operate a relay. The agent will then respond with a GetResponse message indicating the change has been made or an error indication as to why the change cannot be made.

The Trap message allows the agent to inform the manager of an important event. An SNMP Trap is a change–of–state (COS) message—it could mean an alarm, a clear or simply a status message.